The invention relates to an image projection system as defined in the preamble of claim 1. The image projection system can be used for displaying image information from a television receiver or a personal computer on a projection screen.
Such a projection system is known from European patent application EP 467447. The projection system described in this document comprises an illumination system for supplying an illumination beam and an image display system comprising three liquid crystal image panels for modulating the light beam in conformity with image information to be projected. The illumination system comprises an integrator system for uniformly distributing the light across the liquid crystal panels. In the known LCD projection system, the light beam having different directions of polarization is separated by a polarization-sensitive beam splitter into two separate light sub-beams having directions of polarization which are perpendicular to each other. The polarization conversion means converts the direction of polarization of one of the two light sub-beams into the other direction of polarization and recombines the converted light sub-beam with the unconverted light sub-beam. Subsequently, the recombined light beam is divided into three further light sub-beams in three respective primary colors by dichroic mirrors. The image panel may be a liquid crystal panel comprising two optically transparent plates between which a liquid crystal layer is provided. Furthermore, the liquid crystal panel is provided with a two-dimensional array of pixels which are driven by means of, for example electrodes. The present configuration of the illumination system comprises a cascade of the polarization-sensitive beam splitter, the polarization conversion means and color separation means. The present configuration occupies a relatively large space in the projection system. However, the market demands lighter and more compact projection systems.
It is an object of the invention to provide an image projection system in which the space occupied by the illumination system is reduced. This object is achieved by the image projection system according to the invention as defined in claim 1. In this configuration, the spaces occupied by the color separation means, the polarization conversion means and the polarization-sensitive filter means may coincide or at least overlap. Therefore, this design occupies less space than the cascade configuration of separate polarization-sensitive filter means, polarization conversion means and separate color filter means as in the conventional projection system. A further advantage is that, when dichroic mirrors are applied as color separation means, the dichroic filters can be used to function optimally by situating them perpendicularly to the paths of the first and second light beams. Further advantageous embodiments of the projection system according to the invention are defined in the dependent claims.
A particular embodiment of the projection system according to the invention is defined in claim 2.
A further embodiment of the projection system according to the invention is defined in claim 3. In conventional projection systems, the angle of incidence of the light beam on the dichroic mirror varies across the illumination window of the dichroic mirror because the spectral properties vary across the illumination window of the dichroic mirror a color shift will then be produced in the illumination window of the image panel. When the dichroic mirror is situated perpendicularly to the light beam, a color shift will not be produced in the illumination window of the first and second image panels.
A further embodiment of the projection system according to the invention is defined in claim 7. Such a tunnel system has the advantage that the cross-section of the first light beam never becomes larger than the illumination window. The illumination window is defined as the dimensions of the cross-section of the light beam at the position of the image panel.
A further embodiment of the projection system according to the invention is defined in claim 8. For example, three prisms may be used and an air gap may be applied as the thin layer of material between the facing planes of the prisms to ensure total internal reflection of the light beam. When the light beam in the optical tunnel is telecentric and the tunnel has a mirror symmetry along the optical axes in a first and a second dimension, respectively, the light beam traverses such an optical tunnel while at each cross-section perpendicular to the light beam, the light beam remains geometrically identical.
A further embodiment of the projection system according to the invention is defined in claim 11. This embodiment provides a compact design for a three-image panel projection system.